Tuning of the resonant cavities within a klystron is accomplished by adjusting the inductive shorting bars which are located within the resonant cavities. The adjustment of the shorting bar within each resonant cavity determines its resonant frequency. The pass band frequency of the klystron is then determined by the resonant frequency of the series of resonant cavities. The resonant cavities are within a vacuum environment within the klystron. The shorting bars are attached to plungers which enter the resonant cavities through hermetic bellows. Thus, by pushing or pulling on the plungers, the position of the shorting bar attached thereto will change within the resonant cavity.
An external klystron band selector is a device which tunes the klystron to a preselected frequency band or frequency channel by precisely positioning the plungers, which in turn precisely position the inductive shorting bars. The band selector allows for repeatedly tuning the klystron to preselected frequency channels within a band of frequencies. An example of a prior art is shown in Thiem, et al., U.S. Pat. No. 4,546,325, issued Oct. 8, 1985 ("the Thiem patent"), which has been assigned to the assignee of the present invention.
The Thiem patent discloses an electromechanical device that is driven by two solenoids. One is a linear acting solenoid, and the other is a limited rotation stepper solenoid. Power is applied to the solenoids only while changing channels. A cylinder or barrel is fitted with set screws which function as cams. The cams are set in rows parallel to the axes of the cylinder. Each of the cams is associated with a respective one of the above-mentioned plungers. Several rows of cams are spaced around the cylinder. Each row is adjusted for a different channel by adjusting the extension of each of the cams in that row.
In the operation of the device disclosed in the Thiem patent, the linear solenoid is first activated to move all the plungers away from the cams. Next, the stepper solenoid is then used to rotate the cylinder so that the desired row of cams is positioned over the plungers. Subsequently thereto, the linear solenoid releases the plungers to rest on the cams. Stops may be provided on the tuner to prevent damaging the plunger if the cam associated therewith is not adjusted properly.
As disclosed in the Thiem patent, the changing of the channels of the klystron may be accomplished relatively rapidly. For example, for a six channel band selector, any new channel is selectable within two seconds. A new channel, tuned by selecting an adjacent row of cams from the present row, may be selected in less than one second.
The prior art device disclosed in the Thiem patent has several disadvantages and limitations. For example, the disclosed device is useful with only certain types of klystrons, these being of a type which have low friction between the plungers and the walls of the cavity. The force vs. stroke characteristics of electric solenoids are usually not sufficient to overcome the spring forces and cavity friction present in many klystrons. Also, the disclosed prior art device has no means for manually tuning the klystron in the case of a power failure to the band selector. Finally, the disclosed prior art device has no means for insuring that the plungers are disengaged from the cams before the cylinder is rotated to a new position.